Mode-locked short pulse laser resonators (oscillators), in particular femtosecond laser resonators, are well known in the art, cf. for instance U.S. Pat. No. 5,079,772 A. The laser radiation generated by such laser resonators is a broad-band radiation, for instance in a wavelength range of about 650 nm to about 1050 nm. It is also known to use laser amplifiers in combination with such laser resonators, and such a laser amplifier is, for instance, disclosed in D. Strickland, G. Mourou, “Compression of amplified chirped optical pulses”, Optics Communications, 55 (6), 15 Oct. 1985, p. 447-449, the disclosure of which being incorporated herein by reference. One problem encountered with such mode-locked short pulse laser resonators is the stabilization of phase and frequency, and such stabilization concepts are dealt with in U.S. Pat. No. 6,785,303 B1 as well as in U.S. Pat. No. 6,724,788 B1. In particular, for phase matching, separate, relatively complicated interferometer-type units are suggested.
On the other hand, U.S. Pat. No. 3,772,609 A proposes a laser cavity configuration having a dual output system. More in detail, there is disclosed a laser with two output coupling mirrors in connection with a laser cavity structure containing an active laser medium and producing two distinct laser frequencies (wavelengths). One end mirror is highly reflective at the first of these two wavelengths, and is partially transmissive at the second wavelength; the other end mirror is highly reflective at the second wavelength, and is partially transmissive at the first of the two wavelengths. Accordingly, the first mirror is arranged to couple out a laser beam having the second wavelength, and to reflect the first wavelength, whereas the second mirror is arranged to couple out the first wavelength and to reflect the second wavelength. The basis for the generation of such two laser wavelengths is that the laser cavity, for instance a plasma tube having Brewster's windows as end mirrors generates such two discrete laser wavelengths. The different laser wavelengths may be used for different purposes, and this without switching over the laser resonator from one mode of operation to another.
Contrary to such a dual frequency laser system, a usual mode-locked short pulse laser resonator produces broad-band laser radiation, as mentioned above. Nevertheless, there are applications conceivable where laser radiation with different properties would be useful, and where it is desirable not to have to switch over from one mode of operation to another, as for instance at the laser arrangement disclosed in U.S. Pat. No. 7,172,588 B2, where it is possible to use a short pulse mode of operation in which laser radiation having a lower power is generated in comparison with another short pulse mode of operation. In particular, it would be desirable to use laser radiation with different properties in a laser arrangement with a phase stabilization device, and in particular in a laser arrangement having coupled a laser amplifier to a resonator.